Bull. Jpn. Soc. Fish. Oceanogr. 70(4), Page 229-239, 2006   Estimation of hatching period and transport processes of juvenile jack mackerel (Trachurus japonicus) and young Japanese common squid (Todarodes pacificus) distributing in the Kuroshio-Oyashio Transition Zone Itsuka Kon1*, Akihiko Yatsu2†, Hiroshi Nishida3, Masayuki Noto3 and Ken Mori2 1 Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, 4-5-7, Konan, Minato-ku, Tokyo, 108-8477, Japan 2 Hokkaido National Fisheries Research Institute, Fisheries Research Agency, 116 Katsurakoi, Kushiro, Hokkaido, 085-0802, Japan 3 National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa-ku, Yokohama, Kanagawa 236-8648, Japan † e-mail: yatsua@fra.affrc.go.jp We described spatial distribution, length frequency compositions and hatching date distributions of juvenile jack mackerel and young Japanese common squid collected from the Kuroshio-Oyashio Transition Zone (KOTZ) in May 2003. Numerical model experiments incorporating geostrophic and wind-driven currents confirmed transport routes of larvae and juveniles of jack mackerel and common squid from their reported major spawning grounds in the East China Sea (and from southern Japan for jack mackerel) to KOTZ. Spatial distribution and length frequency data and numerical transport model results of common squid suggested their northward migration during spring. Jack mackerel distributed in the KOTZ may not recruit to fishing grounds in the central and northern Japan, since their northward migration was not evident and jack mackerel catch in the northern Japan was much smaller than that of common squid. Key words: Japanese common squid, jack mackerel, Kuroshio-Oyashio Transition Zone, recruitment, transport